This invention relates to blood sampling devices and more particularly to blood collection devices having anti-backflow valve means.
Evacuated containers or tubes having needle-pierceable stoppers are used extensively in drawing blood samples for clinical testing. A conventional method of obtaining a sample is to employ a blood collection tube, and a tube and needle holder having a double-ended needle cannula. After the distal end of the cannula is placed in the vein of a patient, the tube is moved in the holder until the proximal end of the needle passes through the stopper. The negative pressure in the tube facilitates the drawing of the blood from the vein of the patient. The filled collection tube is subsequently centrifuged to centrifugally separate the blood into its relatively light phase, serum or plasma, and its relatively heavy cellular phase. After phase separation, the light phase is removed for testing. Where serum is to be separated, a blood clot is allowed to form before centrifugation.
When employing conventional collection tubes, faulty techniques in drawing blood can cause drawn blood to be returned to the patient. For example, if the content of the tube is allowed to contact the proximal end of the needle and a tourniquet is not removed soon after blood begins to flow or if the arm of a patient is raised, a drop in venous pressure may cause the backflow of blood from the tube to the patient. Also, if a force is applied to the tube in a manner to compress the stopper against the tube holder while the tube contains blood, a pumping effect may be produced causing withdrawn blood to flow back to the patient. It is, of course, important to prevent the flow of blood drawn back into the patient especially where non-sterile collection tubes are used or where reagents or chemicals are used in the tube for specific test purposes.
Valves have been provided in the needle assembly associated with the tube holder for preventing the backflow of drawn blood to the patient. U.S. Pat. No. 3,874,367, for example, shows a valve disposed in a needle assembly. This construction, however, is relatively expensive since it requires two needles, two hubs, and the steps of securing each needle to its hub.
It is also known to employ a valve in the tube and connected to the tube stopper for preventing the backflow of blood from the tube to the patient. For example, devices having such anti-backflow valves are shown in U.S. Pat. Nos. 4,112,924 and 4,134,512. Such valves generally have a valve chamber into which blood flows from the needle, and a pressure responsive valve member which opens the valve when the pressure in the valve chamber is greater than the pressure in the evacuated tube to thereby allow the blood sample to flow into the tube. Should the fluid pressure in the tube become greater than that in the vein under certain conditions, the valve will prevent backflow of blood from the tube to the patient.
One disadvantage of using such anti-backflow valves of the type shown in the above patents is that some blood remains in the valve chamber after the tube is filled and it is possible or even likely that the valve will open and allow the flow of such blood into the sample sometime during phase separation due to centrifugal forces acting on such blood and the valve. Such blood (or blood clot) could then contaminate the light phase of the blood sample resulting in inaccurate or unreliable blood test results.
In copending application Ser. No. 810,257, filed June 27, 1977, and assigned to the same assignee as this application, a blood collection device is disclosed having a valve constructed such that a blood clot trapped in the valve chamber after the device is filled with a blood sample will tend to be trapped in a narrow passage of the valve so as to reduce the chance of it contaminating serum. However, it is possible under certain conditions that some sample contamination may occur with that device.